Hi Gareth,
Thanks so much for your help.
1. Cron runs every minute
2. Theme Designer mode is not on
3. No right to left languages installed
4. There's 44 additional plugins installed and enabled (I've uploaded as an excel file)
5. Approximately one month after our moodle upgrade
6. Yes a 3rd party helped us upgrade to 3.5 back in June. 1 month later the disk was full due to this cache issue and it has been filling up on average about once a month since then (after we clear the theme cache)
7. I haven't tried switching themes. We aren't actually using Boost, but I will find out from our Moodle admin if I can try temporarily switching themes. Perhaps as you suggest it is because the current theme is a child of Boost. I didn't think it was as it doesn't have a note "Required by: " in the plugins overview, but I don't have enough knowledge to know if that is even relevant.
8/9. We don't currently have a test server, so we haven't tried this yet.
Ok, I definitely won't uninstall Boost! Thanks again. I really appreciate your time.